rcu: Prevent force_quiescent_state() memory contention

Large systems running RCU_FAST_NO_HZ kernels see extreme memory
contention on the rcu_state structure's ->fqslock field.  This
can be avoided by disabling RCU_FAST_NO_HZ, either at compile time
or at boot time (via the nohz kernel boot parameter), but large
systems will no doubt become sensitive to energy consumption.
This commit therefore uses a combining-tree approach to spread the
memory contention across new cache lines in the leaf rcu_node structures.
This can be thought of as a tournament lock that has only a try-lock
acquisition primitive.

The effect on small systems is minimal, because such systems have
an rcu_node "tree" consisting of a single node.  In addition, this
functionality is not used on fastpaths.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

1 files changed, 1 insertions, 0 deletions

diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 7fb93cedc76a..8f0293ce1517 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -202,6 +202,7 @@ struct rcu_node {
                                 /*  per-CPU kthreads as needed. */
         unsigned int node_kthread_status;
                                 /* State of node_kthread_task for tracing. */
+        raw_spinlock_t fqslock ____cacheline_internodealigned_in_smp;
 } ____cacheline_internodealigned_in_smp;
 
 /*